This invention relates to a new process for treatment of negatively charged high surface area particulate filter material to improve the filtration efficiency for small particle, especially submicron particles, and the improved filter media obtained thereby.
The filtration of fine particle size contaminants from liquids has been accomplished by the use of various porous filter media through which the contaminated liquid is passed. With particulate filter materials, the filter media often takes the form of a porous cake or bed of the particulate material deposited on a porous support or substrate. To function as a filter, the media must allow the fluid, commonly water, through while holding back the particulate contaminant. This holding back of the particulate contaminant is accomplished by virtue of the operation, within the porous media, of one or both of two distinctly different filtration mechanisms, namely (1) mechanical straining and (2) electrokinetic particle capture. In mechanical straining, a particle is removed from the fluid stream by physical entrapment when it attempts to pass through a pore smaller than itself. In the case of the electrokinetic capture mechanism, the particle collides with a surface within the porous material and is retained on the surface by the attractive short range van der Wall's type forces.
In those particulate filter media that depend upon mechanical straining to hold back particulate contaminants, it is necessary that the pore size of the filter medium be smaller than the particles size of the contaminants that are to be removed from the fluid. If it is desired to remove suspended solids of a fine particle size with a particulate filter medium that functions by mechanical straining, the cake or bed formed by the filter material need have correspondingly small pores. Such a filter medium normally exhibits low flow rates and a tendency to clog rapidly.
In those filter media that function by virtue of the electrokinetic particle capture mechanism, it is necessary that the filter medium have such a small pore size. The ability to achieve the required removal of suspended particulate contaminants with a filter medium of significantly larger pore size is attractive inasmuch as it allows higher flow rates and reduces the tendency of the filter medium to clog rapidly. The ability of a filter medium to hold back suspended solids of a fine particle size by electrokinetic particle capture depends, to a significant extent, on the surface properties, particularly the surface charge, of both the filter material of which the medium is composed and the suspended solids. It is not feasible to measure the charge on a surface directly. Instead, various indirect techniques, such as electrophoretic mobility, streaming potential, etc., are used to determine the zeta potential, i.e., the electric potential excess of the surface, and the surrounding fluid to the hydrodynamic shear plane, over the bulk potential of the fluid. Inasmuch as surface charge can only be quantified in terms of the zeta potential, all further characterizations of surface charge will be in terms of the zeta potential. While the zeta potential exhibited by a surface normally depends upon the composition of the filter material, it may be modified by other materials that become adsorbed or chemically bonded to the surface of the filter material.
The effect of zeta potential on the electrokinetic capture mechanism is associated with the ability of the suspended solid contaminant particle to come into contact with a surface within the proous filter medium. In order for such contact to occur, it is necessary that either the contaminant particle or filter material surface possess a zero zeta potential, or that the surface have an opposite zeta potential from that of the contaminant particle. If the contaminant particle and the filter material surface have like zeta potentials, there will be a repulsive effect that interferes with the particle's ability to come into contact with the surface. Once in contact with the surface, the particle will be retained by short range van der Waal's forces, which are always attractive. In those situations where electrokinetic capture does occur, the filtration performance is enhanced by the availability of a high filter material surface area within the filter medium. The reason for that is that, as the oppositely charged particles deposit on the surface of the filter material, the deposited particles tend to modify the zeta potential of the surface and, eventually, the surface will develop a like zeta potential, effectively inhibiting any additional deposition. The availability of a high opposite charge and a high surface area thus extend the functional life of the filter medium. The particle size, and the geometry, porosity and depth of the filter medium also affect the life and filtration efficiency.
Asbestos fiber has long been used for filtration of fine or very fine solids and the use thereof has been welldocumented in the literature, e.g. Proceedings of the Filtration Society, Filter Sheets and Sheet Filtration by Geoffrey Osgood, published in Filtration and Separation, July/August 1967, pp. 327-337 (A paper originally presented at the Filtration Society Meeting, London, Apr. 4, 1967), also, Proceedings of the Filtration Society, Asbestos Filter Sheets by D. McLean Wyllie, published in Filtration and Separation, /March/April 1973, pp. 175-178 (A paper originally presented at the second joint AICHE-Filtration Society Symposium, Minneapolis, Minn., Aug. 27-30, 1972). The high filtration efficiency of asbestos fibers is attributable not only to mechanical straining effects but also to the fineness and high surface areas of the fibers, which, in conjunction with the positive zeta potential exhibited by asbestos, result in a highly efficient electrokinetic capture of negatively charged contaminant particles. Attempts to duplicate the filtration efficiency of asbestos in other materials by duplicating the physical state (size, shape, etc.) of asbestos fibers have, up to the present, not been effective. For example, duplication of asbestos filter material with glass fibers, or diatomaceous earth, has not provided filtration efficiency even approaching that of asbestos. No other filter material has a positive zeta potential of the same order as asbestos fiber.
In the past, improvement in the filtration efficiency of filter material has taken several approaches. Improved filtration performances have been realized by chemical treatment to alter the surface properties and, thereby, the size of the suspended solids (i.e., flocculation) to improve the straining effects. (See U.S. Pat. Nos. 3,131,144; 3,227,650; 3,325,492; 3,297,106; 3,542,674; 3,562,154 or 3,668,184). Improved filter performance is also realized by altering the surface properties of the filter material. (See U.S. Pat. Nos. 2,040,818; 2,036,258; 2,797,163 or 2,971,907).
Development of the art has favored the application of specific highly charged polyelectrolyte materials. Thus, U.S. Pat. Nos. 3,352,424 and 3,242,073 describe the coating of filter materials with organic polyelectrolytes, e.g. polyalkylenimines. Baumann et al, in "Polyelectrolyte Coatings for Filter Media" Proceedings of the Filtration Society: Filtration and Separation (Nov./Dec. 1970) pp. 652-690 describe additional research.
These polyelectrolyte materials are unfortunately of relatively sophisticated structure, high molecular weight, high charge density and concomitantly have high manufacturing cost. Accordingly, advantage is seen in the provision of a cheaper but efficacious substitute.
An improvement in liquid filtration as for food and beverage compositions, pharmaceuticals, or in water purification and the like, especially for submicron negatively charged contaminants has now been achieved by modifying the surface of negatively charged particulate high surface area filter materials with a cationic melamine formaldehyde colloid. Surprisingly, the improvement is afforded by treatment with the colloid form of a melamine formaldehyde resin despite its low molecular weight and low specific charge. The resin is applied, as hereinafter described more particularly, to the particulate filter material, and the porous filter medium in the form of a filter bed or cake is subsequently formed dynamically in a manner well-known to the art, as by vacuum techniques.
Melamine-formaldehyde resins including the cationic colloid are well-known and these resins have commonly been used in the paper industry to impart water resistance, i.e. wet-strength, as shown in U.S. Pat. No. 2,563,897. Melamine-formaldehyde colloids are formed from solutions of a melamine monomer acid addition salt by condensation of about 20 units of the monomer with elimination of water to form cationic colloid particles. During formation, some of the acid of the monomer addition salt is liberated and the progress of condensation can be followed by measuring the drop in pH. The chemistry of melamine-formaldehyde colloids is described in numerous literature articles, e.g. Chapter 2, Melamine Formaldehyde, C. S. Maxwell, Wet Strength in Paper and Paper Board, Tappi Monograph Series #29, 1965 and Amino Resins, John J. Blair, pp. 19-25, Reinhold Publishing Corp., New York, 1959, and the specific colloids of the invention may be prepared in accordance with these disclosures or U.S. Pat. Nos. 2,345,543; 2,559,220 or related U.S. Pat. Nos. 2,485,079-80. The compounds generally designated melamine-formaldehyde embrace the methylol melamine monomers in which there are from 1 to 6 methylol substituents, the most commonly employed being the di- or trimethylol compounds.
High surface area particulate filter materials are well-known in the art and include such recognized materials as diatomaceous earth, perlite, sand, etc. These materials are characterized by either the finely-divided state in which they exist, or a fine internal porosity, either of which provides a high surface area per unit volume. The non-porous particulate solids such as perlite and sand are characterized by particles of a size small in comparison to the normally used depth of the filter bed, whereas diatomaceous earth is additionally characterized by a fine internal pore structure. These high surface area particulate filter materials tend to exhibit a negative zeta potential. Consequently, when such material is used for filtration of fine particle suspended solids, there is no particle removal by electrokinetic capture because most suspended solids are also negatively charged. Thus, the only filtering effect attainable with these negatively charged filter materials is the so-called straining effect described hereinbefore. Thus, the efficiency of such filter materials with fine particle suspended solids is limited and never approaches the filtration efficiency of positively charged filter materials known to be useful for this purpose, e.g. asbestos fibers.